Method and equipment for the purification of a liquid

ABSTRACT

Method and equipment for purification of a liquid, where the liquid is polluted by other liquids or solid material, e.g. removing oil from oil contamined water, comprising a flocculation device (2) and a flotation device (8). The method comprises adding one or more chemicals to the liquid in a flocculation device that comprises one or more pipe loops (2) with built-in agitators (3, 20, 21) providing turbulence and plug-type flow through the loop. Thereupon purified liquid and pollutants are separated in the flotation device or in a sedimentation device (8). The equipment according to the invention comprises a pipe loop (2), provided with built-in agitator(s) (3, 20, 21), connected to the flotation device (8).

BACKGROUND OF THE INVENTION

This invention relates to a method and an equipment for the purificationof a liquid, wherein the liquid may be polluted by other liquids orsolid material, e. g. removing oil from water, comprising a flotationdevice.

Extracting oil and gas offshore results in large amounts of sea watercontaining oil. The major part of the oil may be separated from the seawater in separating containers or tanks, but this purification is notsufficient for rendering the sea water back to the sea. Today,authoritative regulations say that spill water from offshore oil- andgas installations shall not contain more than 40 ppm oil.

When removing oil and/or other pollutants from water there are commonlya plurality of methods using centrifuges, hydrocyclones and flotationdevices.

Centrifuges are advanced machines giving a high degree of purification,but their specific capacity is low. The use of a purificationinstallation of this type when extracting oil, comprises a plurality ofmachines connected in parallel. Such installations are very expensiveand it is required to keep a large stock of spare parts. The maintenanceroutines are also quite comprehensive.

Another disadvantage is that installations of this type are ratherenergy consuming. A typical energy consumption is 1 kwh/m³ of purifiedwater.

The hydrocyclone may be considered as "common art" because they arepreferred in new platform installations. These cyclones have quite goodperformances at favourable working conditions. Meanwhile, they have somelimitations and disadvantages. In the presence of small oil droplets, achemical flocculation device has to be installed upstream thehydrocyclone installation. The shear forces that may occur in thehydrocyclones are large, and the flakes have to be very robust. Flakeswith the required strength may be difficult to produce.

Further, the pressure drop in the hydrocyclone is large, commonly morethan 40 m W.P.

Downstream from the hydrocyclone it is required to have a flotationchamber with a rather large capacity when relieving the pressure.According to certain working conditions a hydrocyclone plant may bequite comprehensive to install. The capacities of the hydrocyclones aresmall, and therefore they have to be connected in groups to serve aslarge batteries when higher capacity is demanded.

The third type of separating devices, flotation plants, arecharacterised in that they have low specific capacities and lowpurification effects. The space needed for this type of installation islarge, and they are often difficult to operate. Today, flotation plantsare regarded as "technology of the past", and are of little relevance inoffshore use because of the disadvantages these plants represent.

SUMMARY OF THE INVENTION

The present invention may be regarded as a sort of flotation plant, butit represents a substantial improved solution among these. Thus, theplant according to the invention needs less space and volume, issubstantially lighter and has better purification effects thanhydrocyclone installations.

The invention is characterised in that the purification processcomprises adding one or more chemicals to the polluted liquid containedin a flocculation device. The flocculation device consists of one ormore pipe loop(s) with built-in agitator(s) that effect turbulence andplug-type flow through the loop, followed by separating the purifiedliquid and the pollutants in the flotation device or in a sedimentationdevice.

The equipment according to the present invention is characterised inthat it comprises a pipe loop with built-in agitator in conjunction withthe flotation device.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in detail with reference todrawings that illustrate embodiments thereof:

FIG. 1 is a schematic illustration of a liquid purification plantaccording to the invention.

FIG. 2 is a schematic illustration of an alternative embodiment of theplant in FIG. 1.

FIG. 3 is on an enlarged scale an agitator included in the inventionshown in FIG. 1 and FIG. 2.

FIG. 4 is on the same scale a similar agitator, with alternative drivingmeans.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIGS. 1 and 2, the plant consist mainly of a pipe system ora pipe loop 1 comprising vertical pipe elements 2, a bubble generatorfor instance static mixers 5, and a flotation device or a sedimentationdevice 8.

The liquid to be treated is, as shown in FIG. 1, passed through the pipeelements 2 where the chemical flocculation takes place. The flocculationchemicals and flotation gas are supplied through the connecting line 4.

Each pipe element has a built-in agitator (FIG. 3) comprising a shaft 20that goes through the element, the shaft being provided with propellerelements 21 and a support bearing 22. The agitator is driven by a motor3 at an appropriate speed. The volume of the pipes is adapted to havesufficient time for obtaining flakes of sufficient size.

To diminish the rotation of the liquid in the pipe, the pipe is providedwith stationary guiding vanes 23 between the propeller elements.

The propellers 21 and the guiding vanes 23 are so designed that desiredturbulence is obtained, and such that the liquid moves in plug-type flowwithout the presence of too large shear forces.

Experiments in a pilot plant have shown that the purification effectwhen separating oil from water would be very satisfying, that is, lessthan 5 ppm when connecting the agitator directly to the flotation deviceformed as a flotation tank or -chamber.

However, in dependency of the type of pollutants that may be present,and in some cases, to further improve the purification process, it isrequired to supply air or another gas in an bubble generator asdescribed above.

In the last embodiment, when the flakes are formed, FIGS. 1 and 2, airor gas is supplied through the pipe 1 and under the support of thebubble generator, e. g. as a set of static mixers, bubbles will beformed at desired size and quantity.

The gas required for flotation may alternatively, and preferably withoutany bubble generator applied, be supplied directly to the lastflocculation pipe element. The liquid flow in this pipe element shouldbe upward directed (not shown). By means of the built-in agitator andthe guiding vanes, gas bubbles of suitable sizes are developed.

The static mixers 5 may be arranged in the connecting pipe between theflocculation pipe 2 and the diffusor 6, or in an outer circuit 14 asshown in FIG. 2.

The bubble and flake containing liquid passes through a diffusor 6 thatresults in a remarkable speed reduction of the liquid before it enters aflotation chamber 8.

A coalescer (bubble retriever) 9 is arranged in the floation chamber.The coalescer comprises tilted or horizontally arranged lamellas thatcover the width of the flotation chamber. The lamellas are made ofplates that may be plane or corrugated, arranged at a vertical distanceof for instance 10-100 mm. The lamellas support formation of gas pocketsat their underside.

Flakes that have retrieved gas bubbles pass with the liquid through thecoalescer in a laminary flow, and will successively adhere to thelamellas.

In this manner, when the flakes have grown sufficiently large, they areforced out of the coalescer by the liquid flow, and rise rapidly to thesurface of the flotation chamber. The flotation gas is ventilated by thepipe 12, the purified liquid is drained through pipe 10, and finally,separated flakes among some liquid is removed through pipe 11.

A partition wall 16 is arranged in the flotation chamber to define twoseparate chambers for respectively purified and skimmed liquid.

The length of the lamellas in the coalescer is so dimensioned that eventhe smallest flakes are retrieved. The distance between the outlet ofthe coalescer 9 and the partition wall 16 is adapted to give the flakessufficient time to rise to the surface of the fluid.

Conventional methods may be implemented to control the amount ofpurified and skimmed liquid removed, although these methods will not befurther described here.

FIG. 4 shows an alternative motor to operate the unit, where the liquidflow is ejected through a nozzle 17 to drive a turbine 18 in connectionwith the agitator.

Technical advantages according to the flocculation system:

The flocculation process is optimised by plug-type flow, optimalstirring and turbulence, limited shear forces that may destroy theflakes, adding chemicals at an unlimited number of places at optimaltime periods.

Further, technical advantages according to the flotation plant:

The plant makes it possible to supply gas/air at an optimal amount andbubble size. By means of the diffusor- and coalescer system theflotation is rendered very effective and the surface area of theflotation chamber becomes small): high load m³ /m² h.

Experiment:

An experiment was performed with sea water containing oil, the amount ofwater was app. 4000 l/h and the oil content in raw water app. 1000 mg/l.The average oil droplet size was 5 my and pH in the sea water was 6.2.Two chemicals A and B were added and the reaction time was respectively45 sec. for each chemical. Approximately 0.1 l flotation gas/l of waterwas added, and the process pressure was 1 bar abs. Results from theexperiment show that there was obtained a reduction of oil in watercorresponding to a remainder of oil below 5 mg/l.

The invention as defined in the claims is not limited to the embodimentsdescribed above. Thus, the pipe loop 2 may comprise horizontally ortilted pipes instead of vertically arranged pipes. Further, the bubblegenerator(s) may consist of one or more gas nozzles arranged in a bubblechamber instead of static mixers. Possibly, the bubble generator maydeliver mixed gas/water directly to the flotation device. The lamellasin the coalescer may be corrugated and coated e. g. with PTFE(polytetrafluoroethylene) to avoid oil sticking on the surface thereof.

I claim:
 1. A method for purification of a liquid which is polluted byother liquids or solid material, which comprises adding one or morechemicals to the polluted liquid in a flocculation device, saidflocculation device comprising one or more pipe loops with built-indynamic agitator means for providing turbulence and plug-type flowthrough the loop, subsequent to said flocculation device purified liquidand the pollutants are separated in a flotation device.
 2. The methodaccording to claim 1, wherein the liquid is water polluted by oil.
 3. Anapparatus for purification of liquid which is polluted by other liquidsor solid material, comprising a flocculation device, comprising a pipeloop with built-in dynamic agitator means for providing turbulence andplug-type flow through the loop to cause flocculation of the othermaterial, and a flotation device connected to said pipe loop forseparating the other material from said liquid.
 4. The apparatusaccording to claim 2, wherein the pipe loop comprises vertical pipeelements connected to horizontal pipe sections, each pipe element havingan agitator comprising a shaft driven by a driving device, wherein saidshaft supports propeller elements and goes throughout the element, andthe agitator further comprises stationary guiding vanes.
 5. Theapparatus according to claim 4, wherein one or more pipe elements isprovided with connecting lines for adding one or more chemicals.
 6. Theapparatus according to claim 4 or 5, wherein at least one pipe elementis provided with connecting lines for supply of a flotation gas.
 7. Theapparatus according to claim 3, 4 or 5, wherein a bubble generator isarranged between the pipe loop and the flotation device.
 8. Theapparatus according to claim 3, 4 or 5, wherein a bubble generator isarranged in an external pipe system where one end of the system isconnected to the flotation device and the other end is connected to thepipe loop immediately before the flotation device.
 9. The apparatusaccording to claim 7, wherein the bubble generator comprises staticmixers.
 10. The apparatus according to claim 7, wherein a diffusor isarranged between the flotation device and the bubble generator.
 11. Theapparatus according to claim 3, 4 or 5, wherein the flotation device isprovided with a coalescer.
 12. The apparatus according to claim 11,wherein the coalescer comprises lamellas arranged horizontally ortilted.
 13. The apparatus according to claim 12, wherein the lamellasare corrugated or shaped in a manner that support formation of gaspockets at their underside.
 14. The apparatus according to claim 13,wherein the lamellas are coated with a coating that is oil repellent.15. The apparatus according to claim 14, wherein the coating is apolytetrafluoroethylene coating.
 16. The apparatus according to claim 6,wherein a bubble generator is arranged between the pipe loop and theflotation device.
 17. The apparatus according to claim 6, wherein abubble generator is arranged in an external pipe system where one end ofthe system is connected to the flotation device and the other end isconnected to the pipe loop immediately before the flotation device. 18.The apparatus according to claim 8, wherein the bubble generatorcomprises static mixers.
 19. The apparatus according to claim 8, whereina diffusor is arranged between the flotation device and the bubblegenerator.
 20. The apparatus according to claim 9, wherein a diffusor isarranged between the flotation device and the bubble generator.
 21. Theapparatus according to claim 6, wherein the flotation device is providedwith a coalescer.
 22. The apparatus according to claim 7, wherein theflotation device is provided with a coalescer.
 23. The apparatusaccording to claim 8, wherein the flotation device is provided with acoalescer.
 24. The apparatus according to claim 9, wherein the flotationdevice is provided with a coalescer.
 25. The apparatus according toclaim 10, wherein the flotation device is provided with a coalescer.